U.S. patent number 4,517,516 [Application Number 06/483,351] was granted by the patent office on 1985-05-14 for nmr probe coil form structure.
This patent grant is currently assigned to Varian Associates, Inc.. Invention is credited to Howard D. Hill, Albert P. Zens.
United States Patent |
4,517,516 |
Hill , et al. |
May 14, 1985 |
NMR Probe coil form structure
Abstract
An RF probe for a gyromagnetic resonance spectrometer includes a
coil cage for supporting the probe coil preferably within the cage
formed of a number of rod-like members displaced parallel to the
coil axis and spaced apart from the axis at a constant distance.
Access to the coil through an axial continuous slot 46 or
equivalent eliminates axial discontinuity in susceptibility due to
discrete holes in the coil form.
Inventors: |
Hill; Howard D. (Cupertino,
CA), Zens; Albert P. (Fremont, CA) |
Assignee: |
Varian Associates, Inc. (Palo
Alto, CA)
|
Family
ID: |
23919709 |
Appl.
No.: |
06/483,351 |
Filed: |
April 8, 1983 |
Current U.S.
Class: |
324/318;
324/321 |
Current CPC
Class: |
G01R
33/34053 (20130101); G01R 33/34069 (20130101); G01R
33/307 (20130101); G01R 33/56536 (20130101); G01R
33/34092 (20130101) |
Current International
Class: |
G01R
33/30 (20060101); G01R 33/34 (20060101); G01R
033/08 () |
Field of
Search: |
;324/318,321,319,322 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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191159 |
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Jan 1923 |
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GB |
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193873 |
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Mar 1924 |
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GB |
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232640 |
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Jan 1926 |
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GB |
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274160 |
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Jul 1927 |
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GB |
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369735 |
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Mar 1932 |
|
GB |
|
701894 |
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Jan 1954 |
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GB |
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Primary Examiner: Tokar; Michael J.
Attorney, Agent or Firm: Cole; Stanley Z. Berkowitz; Edward
H.
Claims
What is claimed is:
1. An RF probe for a spun sample gyromagnetic spectrometer
comprising:
(a) sample containment means for holding a sample for analysis,
(b) sample spinning means for rapidly rotating said sample
containment means about a rotation axis,
(c) RF saddle coil means surrounding said sample containment means
for exciting and detecting gyromagnetic resonance in said
sample,
(d) RF coil support means for supporting said RF coil in axial
alignment with said rotation axis and spaced from said axis, said
support means having at least one axially aligned slot in the
lateral wall thereof, the axial length of said slot extending along
substantially the axial length of said RF saddle coil means.
2. The RF probe of claim 1 wherein said coil is disposed within the
interior of said coil support means.
3. The RF probe of claim 2 wherein said RF coil support means
comprises an axially symmetric assembly of rigid axial support
members disposed in the plane transverse to the symmetry axis, each
member parallel to said axis and plate members for securing the
ends of said axial members, whereby an open structure is formed for
supporting said RF coil.
4. An RF probe for a spun sample gyromagnetic spectrometer
comprising an RF coil surrounding a sample region and an axially
symmetric assembly of rigid axial support members disposed on the
circumference of a circle in the plane transverse to the symmetry
axis of said coil, each member parallel to and displaced from said
axis by constant distance, and planar members forming the ends of
said support structure, whereby open structure is formed for
support of said RF coil.
5. The RF coil support structure of claim 4, said rigid axial
support members adapted to receive respective compensating members
in proximity to the end regions of said axial support members, said
compensating members selected to extend the average magnetic
susceptibility of said coil region into said end regions.
Description
FIELD OF THE INVENTION
The present invention relates generally to analytic instrumentation
based upon magnetic resonance phenomena and particularly relates to
reduction of magnetic perturbations due to inherent structure in
the probe of an NMR spectrometer.
BACKGROUND OF THE INVENTION
In the typical nuclear magnetic resonance (NMR) analysis
instrument, a sample is placed within a volume situated within a
homogeneous region of magnetic field. Excitation and detection of
resonance is obtained from a suitably placed coil (or coils)
ordinarily spaced with respect to the sample and preferably
enveloping it. Quite typically, for modern Fourier transform
resonance spectroscopy, the sample is contained within a
cylindrical tube disposed coaxial with, and within, a single coil
and means are provided to rapidly rotate the sample tube about its
axis to average any residual inhomogeneities for the magnetic
field.
The material environment of the sample volume of typical prior art
apparatus may contain a number of substances: the sample container,
usually glass and possibly including a stopper delimiter of nylon
or similar inert material; a conductive material forming the RF
coil conductor, commonly copper, aluminum, silver, gold or platinum
or a combination of these materials; a coil form supporting the
coil; a bonding agent for securing the conductor to the coil form;
one or more holes in the coil form for interconnection of coil
winding components; and, air permeating all available spaces. These
materials distinct from the sample and solvent itself, exhibit
various magnetic susceptibilities and influence the signal by
varying the magnetic field distribution throughout the sample. The
relative rotation of the sample and the RF field acts to average
sources of magnetic perturbations of non-cylindrical geometry
whereby the average produces an equivalent cylindrical symmetry.
Some of these sources have been considered in prior art
compensatory schemes. Coil materials and bonding agent materials
have been considered by Anderson, et al., U.S. Pat. No. 3,091,732
where it was sought to provide coil materials and bonding agents
for securing the coil to a coil form, which materials were required
to exhibit a magnetic susceptibility approximating air, (in which
these components are necessarily submerged). The inhomogeneity due
to structure present within the active volume of the spectrometer
is compensated by fabricating a material which has magnetic
properties identical with the properties of the solvent whereby
axial homogeneity over the sample volume is contained. This is
described in U.S. Ser. No. 482,344. The compensation of the
geometric axial distribution of materials forming a saddle coil is
discussed in U.S. Ser. No. 534,899.
In the prior art the form supporting the RF probe coil is a
cylinder of glass or similar material. There are advantages for
mounting the coil on the internal surface of the form to obtain
closer coupling with the sample but there remains the requirement
to provide access to the coil terminals. Holes may be provided in
the form or else (saddle) coil leads must be directed along the
form with insulation provided at the crossover point which
necessarily occurs with saddle coil structure. Where the conductor
is specially fabricated to exhibit a desired magnetic
susceptibility the axial magnetic discontinuity as introduced by
holes or insulating materials becomes relatively significant. Any
such discontinuity within the active sample region and shorter than
the axial extent of the active region will cause such a significant
perturbation. For a spinning sample the discontinuity can be
averaged azimuthally. If the discontinuity is lengthened in axial
extent, the preferred cylindrical averaging will homogenize the
perturbation and effectively eliminate it.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 schematically illustrates the context of the invention.
FIG. 2 shows a simple example of the principle of the
invention.
FIG. 3 shows a preferred embodiment.
FIGS. 4a, 4b and 4c show an embodiment exhibiting additional axial
compensation.
FIGS. 5 and and 5b compare spectra obtained with and without the
present invention.
BRIEF DESCRIPTION OF THE INVENTION
A simple embodiment of the invention is apparent where a coil form
which requires a hole in the cylindrical wall is furnished instead
with a long slot extending longitudinally. Rotation of the sample
results in averaging of the discontinuity in the azimuthal
direction whereas the length of the slot assures axial continuity.
If the slot is of constant width, the averaged magnetic
susceptibility is independent of axial coordinate.
Cylindrical symmetry for coil support is here obtained in one
embodiment by arranging a cage formed from a plurality of glass (or
like material) rods or tubes, the axes of which lie on the
circumference of a circle in the plane transverse to the coil axis.
Preferably, a saddle coil is disposed within the cage. The
resulting open structure of the cage permits access to saddle coil
leads and provides an average cylindrical symmetry from the
viewpoint of a spinning sample.
In that embodiment wherein the glass cylinders are hollow tubular
members, additional compensation to achieve magnetic homogeneity
can be accommodated in the interior of the tubes. In particular a
helical coil occupying the central region, measured along the axis,
of the active volume of the probe is contained within the desired
structure. To maintain the average susceptibility prevailing in
that central region (due to the coil) over a substantial axial
region beyond the coil, a material of specified magnetic
susceptibility is inserted in each tube to extend from the tube
ends inwardly toward the central region terminating in the vicinity
of the helical coil. The material is selected to provide a
susceptibility, which averaged over the (imaginary) cylindrical
surface, provides axial continuity between the central region and
the adjacent areas.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to FIG. 1 an NMR spectrometer 30 is shown in a
schematic block diagram form to include a high field magnet
symbolically indicated by poles 31 with an air gap into which a
probe 32 is located. Secured to the top of the probe is a spinner
assembly 33 which receives the sample tube, not shown. Spinner
assembly 33 supports the spinning of the sample tube in the
magnetic field, implemented form an air supply 34 connected to the
spinner to provide rotation thereto. An RF transmitter/receiver in
signal processor 35 is connected to the probe 32 which probe
includes coils, not shown, for exciting and detecting resonant
spectra of the sample in the sample tube. The signal processor also
includes means for displaying the spectrum of the sample under
examination as indicated symbolically by display means 36.
FIG. 2 shows a conventional cylindrical coil form 40 for supporting
on the interior thereof a saddle coil, a portion 42 of which is
shown. A slot 44 has an axial extent in excess of the length of the
coil. When a sample in the interior of the coil is spun about axis
46, the magnetic susceptibility of the form 40 and slot 44 is
averaged azimuthally and the average is axially continuous.
Turning now to FIG. 3 there is shown in partial projection a saddle
coil 60 of radius R-r nested within a cage 62 formed from a
plurality of parallel rods or tubular members 64, 66, 68, 70. The
rods or tubes are preferably of glass or ceramic or like material
which does not contribute a resonant signal in the frequency
spectrum under study. These members are spaced apart on the
circumference of radius R and form an imaginary inscribed circular
bound of radius R-r where r is the radius of a tubular member 64,
66, etc. The cage 62 has end plates 72 and 74 which contain holes
bored on the said circle of radius R to receive the rod or tubular
members. The length of the cage is of course long compared to the
actual length of the active volume which is substantially defined
by the axial extent of the RF probe coil.
FIG. 4a shows in transverse section a saddle coil 76 (or
alternately, a helical coil 86) supported by one or more, members
64, 66, etc. A projection from a longitudinal section is shown in
FIGS. 4b and 4c for respectively a saddle coil 76 and a solenoidal
coil 86. The members 64, 66, etc. are preferably hollow tubes into
which compensatory bodies 82 are inserted respectively to axially
extend the average magnetic susceptibility properties of the coil
and coil form structure at least as averaged over a cylindrical
surface of radius R-r. The material body 82 is selected to yield in
the aggregate this desired average magnetic susceptibility. As a
result, the axial continuity in magnetic susceptibility is
established and spectra obtained therewith are characterized by the
improved resolution and effective sensitivity concomitant with a
magnetic field exhibiting reduced inherent gradients. FIGS. 5a and
5b are a comparison of the portion of the proton decoupled 13C
signal of dioxane obtained with a prior art probe featuring a coil
supported within a glass tube of radius 5.4 mm, (FIG. 5a). In FIG.
5b, the coil cage of the present invention supports an identical rf
saddle coil (2.2 cm in length by 5.5 mm radius) and the same
spectral region of the same molecule at the same concentration is
shown to indicate the effect of the present invention. The same
peak is plotted to the same statistical precision (number of
transients). By inspection, the resolution is clearly improved and
the high and low frequency tails of FIG. 5a are clearly suppressed
in the peak of FIG. 5b.
It is to be understood that many changes can be made in this
specifically described embodiments described above without
departing from the scope of the invention, and that the invention
is to be determined from the scope of the following claims, not
limited into specifically described embodiments.
* * * * *